Functionalized Graphene Nanosheets with Fewer Defects Prepared

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Functionalized graphene nanosheets with fewer defects prepared via sodium alginate-assisted direct exfoliation of graphite in aqueous media for lithium ion batteries Mengdan Hu, Yifeng Zhou, Wangyan Nie, and pengpeng chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00518 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Nano Materials

Functionalized graphene nanosheets with fewer defects prepared via sodium alginate-assisted direct exfoliation of graphite in aqueous media for lithium ion batteries

Mengdan Hu, Yifeng Zhou, Wangyan Nie, Pengpeng Chen*

School of Chemistry & Chemical Engineering, Anhui University, Hefei 230601, China Anhui Province Key Laboratory of Environment-Friendly Polymer Materials

*author to whom correspondence should be addressed: Pengpeng Chen School of Chemistry & Chemical Engineering, Anhui University, Hefei, 230601, People’s Republic of China

E-mail:[email protected]

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Abstract: We had been proposed a cost-efficient method to produce graphene nanosheets from graphite by using high shear mixer in aqueous sodium alginate (SA) solution. Results from Transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS) as well as thermal gravimetric analysis (TGA) revealed that the graphite was successfully exfoliated into graphene nanosheets, and the obtained graphene was modified by SA after exfoliation and improved dispersion in water was achieved. The concentration of graphene could attain to 0.60 mg/mL at graphite concentration of 15 mg/mL. When used as nanofiller, a significant increase of 65% and 73% in tensile strength and storage modulus of PVA were achieved by incorporating of only 0.5 wt% graphene nanosheets, respectively. When the exfoliated graphene was applied as electrode materials for lithium ion batteries (LIBs), a high initial reversible discharge capacity of 1450 mAh g -1 was obtained at 100 mA g-1, and the capacitive contribution made up about 20.1% at 1 mV s−1, suggesting the unique structure of graphene nanosheets exhibited excellent electrochemical performance. Keywords: Graphene; Sodium alginate; Reinforcement; Electrochemical performance

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1. Introduction Graphene, with sp2-hybridized carbon atoms1 and π-electron cloud,2 had received tremendous attention just that its large theoretical specific surface area,3 high electrical conductivity4 as well as outstanding chemical inertness.5 Intensive research had been focused on the utilization of graphene-based materials in various potential applications such as supercapacitors,6 composites,7 lithium-ion batteries (LIBs)8 etc. Up to now, the graphene nanosheets had been prepared by micromechanical exfoliation,9 chemical vapor deposition,10 reduction of graphene oxide (GO)11 and liquid-phase exfoliation.12 Among the methods, chemical reduction of graphene oxide boasted the advantage of a great deal of fewer layer graphene but suffered substantial quantities of structural defects and oxygen functionality.13 Recently, obtaining graphene through the exfoliation of graphite in organic solvents,14 aqueous surfactant solutions12 and ionic liquids15 had been considered to be a hopeful low cost for the production of graphene nanosheets. However, the organic solvents were toxic, lack biocompatibility and expensive, as well as the ionic liquids were of high cost.16 Water offered a host of advantages as a solvent-being cheap, abundant, non-flammable and nontoxic, which therefore called the most attentions. However, it was known that the exfoliation of graphite in solution were defiant to avoid aggregation because of the hydrophobic nature of graphene.17 The use of surfactants for exfoliation was important when water was employed as the solvent, due to the manifest increase of dispersion stability of the nanosheets.1 S De et 3



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al. demonstrated a process to prepare graphene nanosheets using sodium cholate (SC).18 Mustafa Lotya et al. dispersed and exfoliated graphite to graphene using sodium dodecylbenzene sulfonate (SDBS) suspended in water-surfactant solutions.19 Min Soo Kang et al. had resoundingly exfoliated graphite into graphene nanosheets by using polyethylene glycol (PEG).20 Whereas some

of the

surfactants exhibited

bioaccumulation and were able of adsorbing to proteins, disrupting enzyme function as well as causing organ damage.21-22 The employment of bio-materials as the surfactant seemed to be more environmental. Some biopolymers possessed amphiphilic character which could successfully stabilize the hydrophobic 2D materials in water.23-24 Polysaccharides were one of the most important types of these biopolymers.25 For instance, cellulose,26 chitosan,27 and lignin28 were constituted long chains of monosaccharide units that were bound together by glycosidic linkages, and they exhibited excellent performance in exfoliating 2D materials in water.29 Sodium alginate (SA) was a low-cost and readily available natural non-toxic hydrophilic polysaccharide derived from brown sea algae,30 as well as a water soluble salt of alginic acid.31 SA was composed of sugar moieties, which contained numerous -OH and -COOH, endowing the

polysaccharide

with

excellent

hydrophilicity

and

water

permselective

nanochannels.30 Therefore, SA was favorable for its intercalation into graphene layers. The aqueous SA solution with low concentration was of high viscosity, which was beneficial for avoiding the exfoliated graphene nanosheets from restack. Considerable studies had been conducted to prepare graphene nanosheets by 4


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surfactant-assisted sonication methods.32-33 However, it was difficult to scale up the processes involving sonication.34 The alternative methods for the exfoliation of graphene nanosheets were significant. In the previous literature, the shear force had shown high efficiency in exfoliating layered materials into thin nanosheets in liquid media.34-36 The high shear mixer had been widely used in pulverization and emulsification in the industry.37-38 The working principles were based on hydrodynamics,39 which was competent to surmount the van der Waals forces between layers.38 In this work, we presented a method to produce high quality graphene in aqueous SA solutions on a large scale and at a low cost by using a mixer (Scheme 1). Interestingly, a large number of SA molecules were strongly absorbed on the graphene nanosheets after the exfoliation and washing. The graphene nanosheets were successfully exfoliated and functionalized at the same time. The reinforcing effect of the obtained graphene was evaluated by taking PVA as a model polymer. The electrochemical property of the graphene was assessed by applying it as the electrode material for LIBs.

Scheme 1. Illustration of the preparation process of the graphene by SA. 5



2. Experimental section 2.1 Materials SA (viscosity: 200~500 mPa.s) was acquired from Adamas Reagent Co., Ltd. (China). Graphite powder (100 mesh, purity ≥99%) were purchased from Shanghai Titanchem Co., Ltd. Sodium nitrate (NaNO3), potassium permanganate (KMnO4), 98% sulfuric acid (H2SO4), 30% hydrogen peroxide (H2O2) and 37% hydrochloric acid (HCl) of analytical grade were acquired from Sinopharm Chemical Reagent Co. Ltd. in China. Hydrazine hydrate aqueous and PVA (degree of polymerization 1750±50, alcoholysis degree 98%, Mw= 75000~80000) were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, china). Deionized (DI) water was used in all experiments. 2.2 Exfoliation of graphene The preparation of the graphene dispersion was described as follows: A desired amount of SA (0.5, 0.8, 1.0, 1.5 or 2.0 g) was added to 100 mL DI water at 50 °C in glass vial with stirring to prepare SA aqueous solution for 2 h. Then, 0.5 g graphite powder was added into the SA solution and dispersed by 0.5 h stirring. After that, the shear time lasted for 4 h by a high shear mixer (FM200, Fluko, China). Eventually, the resulting solution were centrifuged for 10 min at 4000 rpm, discreetly removed and washed seven times with DI water at 12000 rpm for 20 min, and vacuum dried at -50 °C. 2.3 Reduction of GO by hydrazine hydrate GO was prepared according to the modified Hummers method and purified by 6


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dialysis and centrifugation as explained in our previous work.40 Typically, 100 mg GO was dispersed in 200 ml DI water in a 250 ml flask under magnetic stirring. Then, 0.2 ml hydrazine hydrate was added into the suspension and heated to 100 °C for 24 h in an oil bath, under condensate return flow equipment. Finally, the obtained black product, reduced graphene oxide (RGO) nanosheets were washed copiously by DI water consecutively, and vacuum dried at -50 °C. 2.4 Preparation of PVA/graphene hybrid films First, 2 g of PVA was dissolved in 20 ml of DI water by stirring at 90 °C in a beaker. Then, 0.01 g of graphene and RGO were separately sonicated in 20 ml of DI water for 2 h. After, the required graphene and RGO solutions were separately dripped into the abovementioned PVA solutions by vigorous stirring for 12 h at room temperature followed by sonication for 30 min. Finally, the solutions were poured into glass Petri dish and heated to 50 °C in oven for 24 h to form membranes, and heated to 60 °C for 6 h. The content of the graphene and the RGO in the PVA membranes were described as wt% behind the sample. The neat PVA membrane preparation method was similar to the above. These membranes were easily lifted from the glass Petri dishes and were analyzed by different techniques. 2.5 Annealing of graphene for LIBs application The obtained graphene powder was annealed which then was placed in a tubular quartz reactor. The reactor was heated up to 800 °C in an argon atmosphere at 5 °C min−1 for 2 h and this temperature was maintained for a desired time (15~60 min) to allow the 7



graphene to proceed through an annealing process. After annealing, the reactor was cooled to room temperature for further analyses. 2.6 Characterization Ultraviolet-solar absorption spectra were measured on a UV-1800 spectrometer (Shimadzu, Japan). X-Ray photoelectron spectroscopy (XPS) analysis was performed on an X-ray photo-electron spectrometer using Al Kα radiation(ESCALAB 250, Thermo Fisher Scientific Co. Ltd). The micro-morphologies of the samples were observed by transmission electron microscope (TEM, JEM-2100, Hitachi) at an accelerating voltage of 200 kV and scanning electron microscopy (SEM, S-4800, Hitachi) at an accelerating voltage 3 kV. Atomic force microscopy (AFM) images were obtained with DI Nanoscope IV (Veeco, USA). Raman spectra were surveyed using a Thermo Nicolet Almega XR with an excitation wavelength at 532 nm. The X ray diffraction patterns were obtained on a XD-3 diffraction meter with Cu Kα radiation (λ= 1.54178 Å) (Beijing Purkinje General Instrument Co, Ltd). Thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere using a NETZSCH thermo gravimetric analyzer (STA-449F3) at a heating rate of 20 °C/min. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 870 spectrometer. Hydrodynamic size distribution and Zeta potential were determined by using Malvern 3000HS. All the samples for zeta potential measurements were observed at 25 °C and pH value of 7.5. The dynamic mechanical properties of polymer composites was characterized using DMTA (Rheological Scientific, DMTA-IV) in the temperature 8


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range of -60 °C to 160 °C at 2 °C/min. Tensile tests were performed on a tensile testing machine (XLW-500N) at 25 °C. Galvanostatic measurements were performed on a Land Battery Measurement System (Land, China) in the voltage range of 0.01-3 V. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were conducted on an electrochemical workstation (CHI 760D). 3. Results and discussion 3.1 Exfoliation of graphite to graphene The SA concentration (CSA), graphite concentration (Cg) and shear time (ts) were investigated by UV-vis spectroscopy. The Lambert–Beer law (A=αCl, where A was the absorbance, α was the absorption coefficient, C was the concentration and l was the path length) was a significant parameter in characterizing any dispersion.41 The A/l was corresponding to the concentration of the sample, which was an important parameter in characterizing any dispersion. Meanwhile, to find the optimum content of SA, graphite and ts for graphene-exfoliation, we measured the absorbance per unit cell length, A/l. The absorbance at 268 nm was measured for a variety of dispersions versus the SA concentration (Cg =5 mg mL-1). Figure 1A showed the A/l values measured from the dispersions of different CSA. The maximum A/l appeared at SA concentration of 10mg mL-1, rather than increases monotonically with the CSA.42-43 The results indicated that SA solutions of too low or high concentrations were not good for the exfoliation of graphite. Similarly, it was observed that the maximum A/l value raised at graphite concentration of 15 mg mL-1 and shear time of 4 h in Figure 1B and Figure 1C.

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Therefore, in consideration of the low costs and product yield, the samples were prepared with CSA= 10 mg mL-1, Cg = 15 mg mL-1 and ts = 4 h. Under these conditions, the concentration of graphene could attain up to 0.60 mg/mL, and the yield was approximate 4.0%, which was higher than the reported values using SDS, SC, PEG and SDBS.18-20, 44 The number-averaged diameter (Dn) of exfoliated graphene was showed in Figure 1D, and the data was range 200 nm from 900 nm as well as achieved the maximum peak in 480 nm. The RGO was estimated to be in the range of 150~550 nm and distribution centered at 320 nm.45 The range of the hydrodynamic size of the exfoliated graphene was a little wider compared with RGO.

Figure 1. (A) CSA (ts= 4 h, Cg=5 mg mL-1). (B) Cg (CSA=10mg mL-1, ts= 4 h). (C) ts (CSA =10 mg mL-1, Cg = 15 mg mL-1). (D) Hydrodynamic size of graphene.

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Figure 2. SEM images of (A) graphite particles, (B) graphene nanosheets. (C) Typical TEM image of graphene. (D) High-resolution TEM image of graphene. The SEM images of the graphite particles and graphene nanosheets were showed in Figure 2A and 2B. Most pristine graphite particles were characterized as a typical layered structure (Figure 2A). As seen in Figure 2B, the structure of graphite was severely demolished as well as the numbers of layers were obviously decreased. Meanwhile, the graphite powders were intensely exfoliated to the ultrathin graphene sheets which thin, curly and layered, indicating the successful exfoliation of graphite particles.46 TEM characterization was employed to verify the successful exfoliation of the graphite. TEM image showed that the graphite had indeed been exfoliated to few-layer graphene nanosheets (Figure 2C and D). Few-layer graphene sheets could be observed in Figure 2C. The graphene nanosheets were transparent, suggesting a very small thickness.47 It was feasible to find that the graphene form wrinkles and stack together, 11



giving a disordered multilayer structure.48 Additionally, the high-resolution TEM image in Figure 2D revealed the thin ribbon-like morphology of graphene nanosheets on the folded edges and the lattice structures had not been destroyed.49

Figure 3. AFM images of graphene nanosheets and the corresponding height profiles. AFM images were investigated the microtopography and height of the graphene nanosheets (Figure 3). The graphene nanosheets could be considered to less than 5 layers of graphene, which was in accordance with the height observed for layer graphene nanosheets in other literatures.19, 46, 50 In addition, the increase in thickness was influenced by the presence of residual solvent in the graphene nanosheets and the roughness of the mica.38, 50

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Figure 4. (A) The Raman spectra of graphite and graphene. (B) 2D Raman band for graphene and graphite. (C) XRD patterns of graphite and graphene. (D) the zeta potential of SA, graphene and RGO dispersions. As shown in Figure 4A, the graphite showed the G and 2D bands at~1581cm-1 and ~2720 cm−1. While the G, D and 2D bands at ~1578 cm-1, ~1347 cm-1 as well as ~2714 cm-1 could be found out graphene spectrum. The D band was attributed to structural defects, the edge disturbance, as well as the diffusion of structural disorder.51 A mean ID/IG value of the graphene was 0.31, which was much smaller than that of the conventional RGO (1.1~1.5), indicating that there were much fewer defects generated during the preparation of the graphene nanosheets.52 The 2D band shape (~2700cm-1) of the graphene nanosheets had obvious change in comparison to the graphite (Figure 4B), which indicated the graphene nanosheets were constitutive of few-layer.19 In addition, a 13



characteristic shift in the 2D peak position and shape were suggestive of the formation of the few layers graphene,53 and the peak position was forcefully affected by the number of layers and the I2D/G ratio was about 0.7 which could also be the few-layer graphene as reported by previously literature.54 Therefore, we considered that the few-layered graphene could be obtained via direct exfoliation of graphite in the SA solution, and the result of Raman spectra was in accordance with TEM, SEM and AFM measurements. The XRD patterns of graphite and the obtained graphene were shown in Figure 4C. The natural graphite exhibited a basal reflection (002) at 26.6°, which was accordant with the literature data.55 The relative intensity of peaks of graphite was greatly weakened after the exfoliation, indicating the successful exfoliation of graphene.56 There was no shift in the peak position of graphene, indicating that the graphene kept the pristine structure and structure with few defects.57 In contrast, the RGO showed an increase in the d-spacing distance which was because of the large number of defects.58 The zeta potential was a significant parameter to characterise stability of the solution. To explain the instantaneous aqueous stability, zeta potential of the SA, graphene and RGO aqueous dispersions were measured. The results (Figure 4D) showed that the SA and graphene were -60.7 and -35.8 mV, which demonstrated the solution's stability and supported the proposed stabilizing mechanism via electrostatic repulsion.25 The values well exceeded the accepted value for a stable colloid (-25 mV).19 However, the zeta potential of RGO was -21.0 mV. This data was similar to Fan Yang et al.’s 14


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investigation.59 The stabilization in water of the graphene obtained from SA-assisted exfoliation of graphite was proved better than the traditional RGO nanosheets.

Figure 5. (A) The FT-IR spectra of graphite, SA and the exfoliated-graphene nanosheets. (B) The XPS spectra of graphite and the exfoliated-graphene. (C) C1s of graphite and the exfoliated-graphene. (D) TGA curves of SA, graphite and the exfoliated-graphene. Figure 5A showed the FTIR spectra for graphite, SA and graphene in the region from 4000 to 500 cm-1. The peak at 3479 cm-1 in the FTIR spectrum of graphite was the stretching vibration of the O-H, and peak at 852 cm-1 represent the aromatic bendings respectively.60 While peaks of the SA around 3446, 1618, 1417 and 1031 cm−1 were assigned to the stretching of O-H, COO-, COO- and C-O-C, respectively.61 Meanwhile, the spectrum of graphene was similar to graphite and exhibited the peak of 1618 cm-1 15



and peaks at 1417 and 1031 cm-1 of SA. There was no peaks around 1700 cm-1 in the FTIR spectrum of graphene, indicating less oxygen-containing functional groups existing on the graphene nanosheets.48 The results confirm that the conducting polymer was successfully introduced onto the SA surface.62 The XPS analysis was applied to clarify the exfoliated graphene (Figure 5B). It showed a pronounced C1s peak at 284.8 eV and weak O1s peak at 532.8 eV, which was ascribed to the physically adsorbed oxygen according to the previously literature.28 For graphite, a peak at 284.8eV was related to the π→π* transition of the aromatic C–C.63 Figure 5C revealed the three carbons types: a strong peak located at a binding energy of 284.8eV attributed to the sp2 carbon (C=C), one peak at 285.2eV assigned to the sp3 carbon and hydroxyl (C–OH), and the peak at 286.4eV ascribed to oxygen–carbon groups (C–O–C). It may be inferred that there were SA residues in the graphene as a result of the existence of hydroxy and ether units in SA.64 TGA was employed to verify the successful functionalization of the graphene nanosheets. Thermal degradation of the SA, graphite and graphene were shown in Figure 5D. Less weight loss occurred for graphite before 700 °C. While the SA was weight loss between 226 and 700 °C, corresponding to removal of molecular fragments and decomposition of structure.65 In comparison to graphite and SA, the decomposition temperatures of the graphene were higher and about 13% of weight loss, which could be attributed to decompose of the adsorbed SA.25 As the graphene had been extensively washed for several times before application and testing, it could deduce that the SA 16


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chains had been strongly adsorbed on the graphene in process of the shear disperse. 3.2 Mechanical properties of the graphene/RGO in PVA

Figure 6. (A) Tensile stress of the neat PVA, PVA/graphene and PVA/RGO composites. (B) Storage modulus of the neat PVA, PVA/graphene and PVA/RGO composites. Table 1. Mechanical property from tensile testing for the neat PVA, PVA/graphene and PVA/RGO composites.

Polymers

Tensile strength

Young's modulus

Elongation at

(MPa)

(MPa)

break(%)

PVA

31±2

74±2

341±7

PVA/graphene-0.5 wt%

51±3

76±2

387±6

PVA/RGO-0.5 wt%

40±2

66±2

381±8

The tensile tests results of PVA composites were displayed in Figure 6A and Table1. The mechanical performance of the PVA composites were dramatically increased compared with the neat PVA. In PVA/graphene-0.5 wt% composite, the tensile strength increased by ~65% and Young's modulus ~3%, as compared to the PVA. However, with regard to PVA/RGO composite with the same filling ratio, the tensile strength increased 17



by ~29% and Young's modulus decreased by ~11% compared with the neat PVA. Obviously, the reinforcing effects of the graphene nanosheets were superior to the conventional RGO. DMA was farther carried out to value the dynamic mechanical properties of the PVA membranes. The storage modulus was a measure of the stiffness. As seen in Figure 6B, both the PVA/graphene and PVA/RGO composite membranes exhibited much higher storage modulus than the PVA. Specially, the storage modulus of the PVA/graphene-0.5 wt% composite was about 73% higher compared to the PVA at -60 °C, while the PVA/RGO-0.5 wt% composite was about 22% higher. The stiffness of PVA/graphene was higher than that of PVA/RGO composites with the same filling ratio. Combined the tensile tests and the dynamic mechanical analysis results, it could be found that the mechanical properties of PVA was enhanced more due to the addition of graphene nanosheets than the RGO. The stiffness enhancement of PVA caused by the addition of graphene nanosheets was much higher than the RGO nanosheets. The two characteristics of the graphene nanosheets were might attributed to its promising reinforcing effects of PVA. On the one hand, compared with the Hummers’ method, there were much fewer defects generated via the direct exfoliation process of the graphite, which was helpful for the keeping of the high strength of graphene nanosheets.66 On the other hand, abundant SA molecules were absorbed onto the graphene nanosheets after the exfoliation and washing, which was rewarding for their homogeneous dispersion in polymers and improved interfacial interactions between 18


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the graphene and the matrix.67 3.3 Electrochemical measurements of the LIBs with graphene as anode

Figure 7. (A) CV profiles of the graphene composite electrode. (B) discharge/charge curves of the graphene. (C) electrochemical cycling performance and the coulombic 19



efficiency at 500 mA g -1. (D) rate capability of the graphene. (E) Nyquist plots of the cells containing graphene composite electrode after 1 and 100 discharge-charge cycles. (F) CV curves of the graphene after one cycle. (G) Red curve shows the CV curve of the graphene and the shaded region indicates the capacitive contribution, measured at 1 mV s−1. (H) The capacitance-controlled contribution of the graphene. Figure 7A showed the cycling voltammogram (CV) of the graphene composite electrode between 0 V and 3 V (scan rate: 0.1 mVs -1). In the first whole cycle, two reduction peaks were detected at 0.7 and 0 V during the negatively scanning, this means that it had been form a solid electrolyte interphase (SEI) film, the intercalation of Li+ into graphene nanosheets, reversible adsorption on defect and edge sites. Meanwhile, the oxidation peaks at 0.2 and 1.5 were observed, which could be attributed to the extractions of Li+ from the defects in the graphene nanosheets during the positively scanning.68 Whereafter, it was clearly visible that the peak current and the integrated area intensity were unvaried and no capacity loss in subsequent scans. It could be concluded that the CV indicated excellent recycling stability of the obtained graphene.69 A significant application of the graphene was to be used as LIBs anodes. The capacity retention was observed in Figure 7B, which ploted the charge and discharge curves of the graphene at different cycles (1st, 2nd, 5th and 50th), the graphene showed excellent discharge/charge properties between 3.0~0.02 V at a 100 mA g-1. An initial discharge and charge specific capacity of 1450 mAh g-1 and 672 mAh g-1 were achieved, with a Coulombic efficiency of 46%, and the 54% capacity loss could be attributed to the SEI.70 Meanwhile, the discharge capacity decreased to 546 mAh g-1 after 50 cycles potentially on account of the SEI.71 Figure 7C showed the cycling performance and coulombic efficiency of the 20


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graphene at 500 mA g-1 for 400 cycles. The discharge capacity was 1695 mA h g-1 and charge capacity was 862 mA h g-1, with a first Coulombic efficiency of 51%. The graphene still exhibited specific capacity of 318 mAh g-1 after 400 cycles. what's more, the Coulombic efficiency gradually increased after cycles, which revealed a good cyclic performance.70 The excellent cycle performance could be attributed to the stable structure of graphene.72-73 Figure 7D showed the capacity variation with rate current density, revealing good rate capability. As we had seen that the discharge capacity of the electrode gradually decreased. For instance, at current densities of 0.1, 0.2, 0.5, 1 and 1.5 A g-1, the discharge capacity of the graphene were 767, 616, 472, 400 and 335 mA h g-1, respectively. Most importantly, a discharge capacity of 256 mA h g-1 could still be obtained at 2 A g -1 and the capacity variation at each rate remained stable, indicating good stability. The electrochemical impedance spectroscopy (EIS) results were showed in Figure 7E. It was well known that the high-frequency semicircle was attributed to the SEI film,74-75 Which was supported by the Nyquist plots obtained from the EIS. The EIS results of first cycling and 100 cycles were shown by the black as well as red curves, respectively. The diameter of the semicircle for graphene was increased after 100 cycles, which was explained that the SEI on the surface of graphene grows thicker with cycle process.76 To evaluate the capacity contributed by the capacitive behavior, cyclic voltammetry (CV) measurements were managed to gain electrochemistry of the graphene. The CV measurements at different scan rates (1-10 mV S−1) were recorded between 0-3.0 V 21



(Figure 7F). A peak shift could be observed as the scan rate increases, certifying polarization of the electrode. Meanwhile, we used the analysis by Dunn to confirm the capacitive contributions.77 The results shown in Figure 7G indicated that the capacitive contribution makes up about 20.1% at 1 mV s−1. The capacity contribution from the capacitance-controlled process rose to 24.4%, 26.2%, 33.2%, and 44.0% as the scan rate increased to 2, 3, 5, and 10 mV s−1, respectively (Figure 7H), which was indicative of the significant role of capacitive charge storage in the electrode, especially at high scan rates.78 4. Conclusions We had proposed a facile method for preparing the graphene nanosheets from graphite in sodium alginate solutions by using high-shear mulser. Abundant SA molecules were absorbed on the graphene nanosheets strongly after the exfoliation and washing process. There were fewer defects generated of the graphene nanosheets prepared via the direct exfoliation method than that of Hummers’ way. The absorbed SA and fewer defects were both beneficial for its application in reinforcing polymers. Taking PVA as a model polymer, the reinforcing effect of graphene on polymer was evaluated. The tensile strength and storage modulus was increased by 65% and 73% with the 0.5 wt% of graphene nanosheets. The reinforcing effect was proved to be better than that of RGO obtained via reduction of tradition GO. When the graphene was used as the LIB anode material, the contribution from the capacitance-controlled process, which could increase to 44.0% at 10 mV s−1 and the LIBs showed a relatively good capacity for retention. The batteries also exhibited outstanding electrochemical performance and cycling stability.

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Acknowledgement This work was financed by Anhui Provincial Natural Science Foundation (1608085QE106), Scientific Research Fund of Anhui Provincial Education Department (KJ2016A791, KJ2017A030).

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Functionalized Graphene Nanosheets with Fewer Defects Prepared

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